How the drug affects your health

Yes, but how bad depends how much you take. Because it has a heavy tar content, and users tend to inhale heavily, smoking cannabis is at least as unhealthy as smoking cigarettes.

The British Lung Foundation believes smoking three joints a day can cause the same damage to your lungs as a packet of 20 cigarettes.

The other worry is what the drug does to the brain. Almost all doctors and scientists admit cannabis is linked to mental health problems. Those who use the drug are more likely to suffer from anxiety, depression and schizophrenia.

Where's the evidence?

Numerous studies have shown a link between cannabis and mental illness. The largest involved 50,000 army conscripts from Sweden. Those who admitted regularly taking cannabis were six times more likely to develop schizophrenia over the next 15 years.

A Dutch study of 4,000 people found that those smoking more than two joints a week were almost seven times more likely to experience psychotic symptoms in later life.

Professor Robin Murray, of the Institute of Psychiatry in London, has found similar results. He describes the link as "very clear".

So what's the debate?

Although cannabis is associated with mental illness, nobody has proved the drug causes such problems. Some experts believe cannabis users are more likely to suffer from anxiety, depression and schizophrenia anyway - they believe the decision to use mind-altering substances is indicative of other health problems.

Cannabis expert Professor John Zajicek, of the Peninsula Medical School in Plymouth, said: "The question is whether there is a causal link. That has not yet been established." Most experts agree that very occasional use of the drug is unlikely to do long-term harm.

How much is too much?

The problem is that these days, cannabis is much stronger than it used to be. Although the "flower power" generation of the Sixties seems to have got away with regular use, modern varieties of the drug are up to 10 times stronger. The level of tetrahydrocannabinol (THC), which induces the high, has increased from about 0.5 per cent 20 years ago to almost five per cent today. This clearly increases the potential risks.

Studies showing a link with mental illness all involved regular cannabis users. Most scientists define regular use as two or more joints a week.

What about cannabis for medicinal purposes?

Pressure is growing for its legalisation for medicinal reasons. The campaign has been led by multiple sclerosis sufferers, who claim it helps alleviate pain and muscle spasms. If they win over the Government, the drug would be in pill form and would be treated so that users would not experience any high. GW Pharmaceuticals, which is allowed to grow 40,000 cannabis plants at a secret location for research purposes, has developed an effective cannabis-based medicine. Called Sativex, it is awaiting licence.

Any other worrying effects?

There is some concern that cannabis affects fertility. A study on mice last year suggests the substance may upset the male reproductive system, resulting in a drop in testosterone.

Another study involving 12,000 British women suggests smoking it during pregnancy can affect the unborn child.

The average baby born to mothers who used cannabis once a week before and during pregnancy was nine ounces lighter at birth than the offspring of non-users.